Whitehall Spirit skiff. Neatly out of the way when not needed, but ready to be lowered onto her launching trolley at a moment's notice for walking to the nearby waterway. And very stylish to boot.
Cameron lives in the US but the house would not look out of place on the Thames. After the floods this winter, I would not be surprised to see boats slung on the sides of riverside homes from Lechlade to Teddington soon.